Methods and apparatus for performing sonomammography and enhanced x-ray imaging

ABSTRACT

Apparatus is provided that combines mammography equipment with an ultrasonic transducer to generate ultrasonic images of the internal structure of breast tissue that are in geometric registration with a mammogram. The apparatus includes a radiolucent and sonolucent compression plate, and in alternative embodiments, a gantry driven ultrasound transducer or a phased array ultrasonic transducer. Methods are provided for generating a mammogram and a plurality of corresponding ultrasound images without moving the breast between the mammogram exposure and the ultrasound imaging. Methods are also provided for viewing and analyzing the ultrasound images. Apparatus and methods are also provided for enhancing X-ray images obtained from conventional mammographic systems, and with reduced overall X-ray dosage to the patient.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/145,958, filed Oct. 29, 1993, entitled METHODS AND APPARATUSFOR PERFORMING SONOMMAMOGRAPHY.

This invention relates to methods and apparatus for imaging breasttissue employing both X-ray and ultrasound technology to provideenhanced diagnostic capability, and enhanced X-ray imaging. Inparticular, the present invention provides methods and apparatus foraugmenting conventional mammography equipment with an ultrasonic imagingsystem that provides geometrically registered X-ray and ultrasonicfields, and associated equipment which may be used to enhance imaging inconventional X-ray equipment.

BACKGROUND OF THE INVENTION

The use of X-ray technology for providing two-dimensional images ofbreast tissue for diagnosis of carcinoma or other abnormalities is wellknown. X-ray imaging has a number of limitations which are universallyrecognized by radiologists. In particular, X-ray imaging of breasttissue has the inherent limitation that a mammogram provides only atwo-dimensional image of a three-dimensional object. Thus, although apotential area of concern may be indicated on a mammogram, the elevationof the subject area within the breast may be uncertain, leading to abiopsy of broader scope than would otherwise be necessary.

In addition to conventional mammograms, apparatus has been developedthat employs ultrasound technology for breast tissue imaging. Ultrasoundimaging devices display echoes received from a piezoelectric transduceras brightness levels proportional to the backscattered echo amplitude.The brightness levels are displayed at the appropriate echo range andtransducer position or orientation, resulting in cross-sectional imagesof the object in a plane perpendicular to the transducer emitting face.

Previously known ultrasound equipment, in the form of dedicatedultrasound breast imaging apparatus, have met with limited acceptance bythe medical community. For example, Brenden U.S. Pat. No. 3,765,403describes the use of ultrasound technology to provide direct andholographic imaging of breast tissue. That device requires the patientto lie prone on a patient supporting surface while her breast isimmersed in a water-filled tank. Taenzer U.S. Pat. No. 4,434,799describes an alternative device wherein the patient's breast isimmobilized between an ultrasonic transducer and ultrasonic receivingtransducer. Both of the systems described in those patents are dedicatedultrasound systems.

In addition to dedicated apparatus, hand-held ultrasound devices havefound application in performing free-hand examinations. Free-handexamination using a hand-held ultrasound transducer is described, forexample, Mendelson, "Ultrasound Secures Place In Breast Ca Management"Diagnostic Imaging, Apr. 1991, pp 120-129. A drawback of such freehandexaminations, when used to supplement mammography, is the inability toprovide geometric registration between the mammogram and ultrasoundimages. This lack of registration may result in the freehand ultrasoundexamination being directed at a different portion of the breast tissuethan would otherwise have been indicated were geometric registrationpossible.

For example, recent studies have shown that over 10% of the massesdetected with free-hand ultrasound and initially believed to be themammographically detected mass, were subsequently found to representdifferent areas of the breast. Because ultrasound can depict 2-3 timesmore cysts than mammography, the possibility of characterizing amalignant lesion as benign is real.

In addition, the three dimensional shape of the lesions, as reported inHomer, "Imaging Features And Management Of Characteristically Benign AndProbably Benign Lesions, Rad. Clin. N. Am., 25:939-951 (1987) and theincreased vascularity associated with carcinoma, as reported in Cosgroveet al.,"Color Doppler Signals From Breast Tumors", Radiology,176:175-180 (1990), have been suggested to be added to the diagnosticcriteria. Such volumetric spatial registration of the ultrasonic datawith a mammogram cannot be accomplished with previously known ultrasounddevices.

While there is recognition within the medical community of theadvantages offered by ultrasound technology, the construction ofconventional mammography and sonography equipment has preventedcombination of these two technologies. In particular, polycarbonatessuch as Lexan®, are typically used in mammography because of theirtensile strength and transparency to X-ray. These materials areacoustically opaque.

On the other hand, the compression plates used in the conventionalbreast ultrasound devices, for example, Brenden U.S. Pat. No. 3,765,403,are composed of materials such as polystyrene or polyurethane, whichhave insufficient tensile strength for use in mammography equipment.

Because of their high densities, all of the materials potentially usefulfor the compression plates in mammography equipment have relatively highattenuation and reflection coefficients (table 1, below). Thesecharacteristics limit the use of ultrasound to low frequencies (3 MHz orbelow as described in Taenzer U.S. Pat. No. 4,434,799) and shallowdepths. At 10 MHz and a 0.5 to 1 cm round trip path through a typicalcompression plate, the attenuation with most polymers would be 20-50 dB.

For any interface thicker than a quarter wavelength (several hundredmicrons, depending on the nominal frequency and acoustic velocity withinthe material) transmission loss must also be taken into account (whichcould exceed 50 dB). In addition, the impedance mismatch between thebiological tissues, the compression plate and the transducer results inat least a 6 dB loss at each interface, or an additional total loss of24 dB round trip. Since the total dynamic range is no greater than 100dB for a typical ultrasound system, ultrasound imaging throughpreviously known mammographic compression plates would be impossible.

In addition, since the acoustic propagation within the compression plateis substantially different than water or the coupling gel, refractioneffects on each of the emitted waves from the elements of a phasedarray, would severely corrupt the beam forming process that assumes aconstant velocity of 1540 m/sec.

                  TABLE 1                                                         ______________________________________                                                    Attenuation Coefficient                                                                        Impedance                                        Material    (dB/MHz/cm)      (Pa s/m)                                         ______________________________________                                        Polyvinylchloride                                                                         11.1             3.4                                              Polybutane  6.1              3.2                                              Polyacetyl, 2.5-3.3          2.2                                              Polyethylene,                                                                 Polypropylene                                                                 Polyamid (Nylon)                                                                          1.1              2.9                                              Polystyrene 1                2.5                                              Water       0.02             1.5                                              ______________________________________                                    

The lower frequencies used in the previously known ultrasonic deviceswould be inadequate for the diagnostic applications, which currentlyrequire 7-10 MHz transducers, yet this higher frequency requirementwould increase the transmission loss by at least threefold (in dB).While it is possible to generate larger pulses in the transducer in thewater bath approach, the low electro-mechanical efficiency results inheat generation. Placing the transducer directly upon the compressionplate, and as a result in close proximity to the biological tissue,would require even higher energy pulses from each element. The resultingheat generation would cause damage and should be avoided.

Conway, "Occult Breast Masses: Use Of A Mammographic Localizing Grid ForUS Evaluation", Radiology, 181:143-146 (1991) and Brem andGatewood,"Template Guided Breast Ultrasound", Radiology, 184:872-874(1992), describe attempts to achieve spatial registration between amammogram and an ultrasound image by cutting a hole in the compressionplate of the mammography device to insert an ultrasound transducer. InConway et al., a cut-open compression plate with a localization grid wasused to allow acoustic transmission. Using the identical ultrasounddevice, the ultrasound study was performed in free-hand and through thelocalizing grid. Several additional X-ray exposures were needed todetect the lesion, replace the compression plate with the cut-out gridcompression plate, then place the cut-out over the coordinates of thelesion. The grid positioned ultrasound detected 24% more lesions thanfree-hand. Ten percent were misidentified using free-hand ultrasound.None of the lesions were misidentified with the grid-guided compression.

The approach described in the foregoing articles has several practicaldrawbacks. For example, in Conway the patient's breast is marked with anindelible pen to assist the mammographer in repositioning the patient'sbreast on the localization grid after the compression plate is replacedby the cut-open compression plate used with the ultrasound transducer.As noted in that article, even the use of indelible markings on thepatients skin does not absolutely guard against movement of theunderlying breast tissue. In addition, the mammographer had to bepresent during the exam to ensure correct positioning, and the procedurelength was significantly increased.

A cut-open compression plate with a localization grid suffers from theproblem that the ultrasonic field is interrupted by the shadow of thecompression plate, in all regions but the cut-out hole, therebyrequiring prior knowledge of the interrogated lesion. As a result, inorder to obtain a complete ultrasonic diagnostic image of the desiredregion of interest, it would be necessary to carry out a complex andburdensome manipulation of the mammographic compression procedure, andexpose the patient to additional ionizing radiation.

In addition to the foregoing, compression plates used in conventionalX-ray mammography typically compress most of the breast mass to auniform thickness. The amount of X-ray exposure needed for imaging isthen determined by the uniform thickness of the tissue between theplates. The nipple region and the outer edges of the breast undercompression have thicknesses that vary widely from the uniformthickness. Thus, the amount of radiation required to properly expose thetissue of uniform thickness causes the nipple region and outer edges ofthe breast to be highly overexposed. To obtain an acceptable image ofthe outer edges and nipple region of the breast, it is typical for theradiologist to perform a second, lower dose, X-ray exposure.

Yet another drawback associated with previously known compression platesis the patient discomfort resulting from the force applied to the breasttissue to compress the tissue to a uniform thickness.

In view of the drawbacks of previously known breast imaging apparatusand methods, it would be desirable to provide an apparatus and methodsfor providing geometrically registered X-ray and ultrasound images ofbreast tissue.

It would further be desirable to provide a compression plate that isboth radiolucent and sonolucent, so that both a mammogram and ultrasoundimages of a patient's breast tissue may be obtained without moving thebreast between the X-ray exposure and ultrasound imaging.

It also would be desirable to provide an apparatus for moving anultrasound transducer through a predetermined path to generate aplurality of ultrasound images of breast tissue at preselectedintervals.

It would also be desirable to provide an apparatus for maintaining alubricating and acoustically coupling fluid film between an ultrasoundtransducer and a compression plate to minimize attenuation andreflection of acoustic energy.

It would be still further desirable to provide an apparatus capable ofcorrelating geometrically registered X-ray and ultrasound images toprovide holographic views of a patient's breast tissue.

It would further be desirable to provide an apparatus for use withconventional mammography equipment which would enhance imaging of thenipple region and outer edges of the breast so that a high quality imagecould be obtained with a single X-ray exposure.

It would further be desirable to provide apparatus for use inconventional mammography equipment which would reduce patient discomfortcaused when compressing the patient's tissue to a uniform thickness.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide an apparatus and methods for providing geometrically registeredX-ray and ultrasound images of breast tissue.

It-is another object of the invention to provide a compression plate foruse in combination mammography/ultrasound (hereinafter"sonomammography") apparatus that is both radiolucent and sonolucent, sothat both a mammogram and ultrasound images of a patient's breast tissuemay be obtained without moving the breast between the X-ray exposure andultrasound imaging.

It is a further object of the present invention to provide an apparatusfor contacting an ultrasound transducer to a compression plate forproviding ultrasound images of breast tissue at preselected intervals.

It is a further object of the present invention to provide an apparatusfor maintaining a lubricating and coupling film between an ultrasonictransducer and a compression plate.

It is a further object of the invention to provide radiolucentultrasound transducer apparatus for use in sonomammography apparatus, toprovide a plurality of ultrasound images of breast tissue that are ingeometric registration with a mammogram obtained by the equipment.

It is a further object of the invention to provide methods for digitallymanipulating ultrasound images of breast tissue, both individually andin conjunction with mammographic views, to isolate and diagnosepotential tissue abnormalities.

It is a still further object of the invention to provide an apparatuscapable of correlating geometrically registered X-ray and ultrasoundimages to provide holographic views of a patient's breast tissue.

It is a still further object of the present invention to provide anapparatus for use with conventional mammography equipment that enhancesimaging of the nipple region and outer edges of the breast to provide ahigh quality image using a single X-ray exposure.

It is yet another object of the present invention to provide apparatusfor use in conventional mammography equipment that reduces patientdiscomfort caused when compressing the patient's tissue to a uniformthickness.

These and other objects of the invention are accomplished in accordancewith the principles of a first embodiment of the invention by providinga radiolucent and sonolucent compression plate that enables sonographyapparatus to be combined with conventional mammography equipment. Eitherbefore or after the X-ray exposure, a carriage mounted ultrasoundtransducer is translated in increments across the compression plate togenerate a plurality of sectional views of the breast tissue. The X-rayand ultrasound images produced by the sonomammography apparatus of thepresent invention are therefore in geometric registration. Those imagesmay in turn be processed by a conventional microprocessor-basedworkstation to provide holographic views of the internal features of apatient's breast.

The compression plate in accordance with the present invention mayinclude a gel pad for acoustically coupling the outer edges of thebreast and nipple region with the transducer. This gel pad may also beadvantageously individually used in conjunction with conventional X-raymammography equipment to provide enhanced X-ray imaging by attenuatingthe incident X-ray radiation proportionally to the tissue thicknessbeing imaged and by reducing the scattering of the X-ray radiation. Thegel pad of the present invention also advantageously enhances breastpositioning and reduces patient discomfort relative to conventionalcompression plates.

In a second embodiment of the present invention, a radiolucentultrasound transducer is provided which is adapted to conventionalmammography equipment. The transducer of the present invention, whichmay be a phased array, serves as both the sending and receivingultrasound transducer, and is positioned beneath the diffraction gridtypically found in mammography equipment for reducing exposure of theX-ray film by scattered radiation. The diffraction grid is modified tofunction as the component of the acoustic circuit in this embodiment.

In yet a third embodiment of the present invention, an ultrasoundtransducer is mounted on a movable carriage positioned between thecompression plate and the diffraction grid of conventional mammographyequipment. For this embodiment, neither the sonolucent compression plateof the first embodiment, nor the radiolucent ultrasound transducer ofthe second embodiment, is required.

The present invention also includes methods of imaging a patient'sbreast tissue using mammography and sonography equipment to providegeometrically registered images. The methods further include processingof those images using a conventional microprocessor based workstation topermit image-guided biopsy of the patient's tissue. Alternatively, themedical practitioner can perform detailed review of the processed andstored images in an off-line setting.

The present invention further includes methods of manipulatingultrasound images, either individually or in conjunction withmammographic views, to assist the practitioner in identifying anddiagnosing potential tissue abnormalities. For example, applicants havediscovered that tissue abnormalities are less compressible than healthytissue. Consequently, applicants have discovered that by performingmultiple ultrasound scans of a tissue mass under different compressiveloads and then digitally subtracting the images, the tissueabnormalities can be readily detected.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments, in which:

FIG. 1 is a perspective view of a first embodiment of thesonomammography apparatus of the present invention;

FIG. 2 is a partial elevation side view of the sonomammography apparatusof FIG. 1;

FIGS. 3A and 3B are, respectively, a side view of a breast compressed inconventional mammography apparatus and an X-ray image obtained with suchapparatus;

FIGS. 4A and 4B are, respectively, a side view of a breast compressed inmammographic apparatus including the gel pad of the present inventionand an X-ray image obtained with such apparatus;

FIG. 5 is a detailed perspective view of one embodiment of a compressionplate in accordance with the present invention;

FIGS. 6A and 6B are, respectively, a perspective view of an illustrativeultrasonic transducer lubricating/coupling device of the invention and across-sectional view of the device of FIG. 6A taken along view line6B--6B;

FIG. 7 is a schematic view of an illustrative embodiment of the drivemeans employed in the sonomammography apparatus of FIG. 1;

FIG. 8 is a perspective view of a workstation and digitizing tabletadapted for use with the present invention;

FIG. 9 is a perspective view of an alternative embodiment of thesonomammography apparatus of the present invention;

FIG. 10 is a cross-sectional view taken along view line 10--10 of FIG.9;

FIG. 11 is a perspective view of the diffraction grid and ultrasonictransducer apparatus of the present invention;

FIG. 12 is a cross-sectional view of another alternative embodiment ofthe present invention;

FIG. 13 is a block diagram of the elements of an ultrasonic imagingsystem in accordance with the present invention;

FIG. 14 is a perspective view of the ultrasonic images and X-ray imagegenerated with the apparatus of FIG. 1 in accordance with the methods ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a first illustrative embodiment ofsonomammography apparatus 10 constructed in accordance with the presentinvention is described. Sonomammography apparatus 10 comprises base 11,vertical column 12, X-ray tube 13 suspended from arm 14, compressionplate 15, ultrasound transducer 16 supported from gantry 17, gantrysupport 18, diffraction grid 19, film holder 20 and biopsy needle guide21.

The mammography components of sonomammography apparatus 10, that is,base 11, column 12, X-ray tube 13, arm 14, diffraction grid 19 and filmholder 20 may include the features hereinafter described, but otherwisemay be conventional. As in previously known mammography equipment, thevertical elevation of arm 14 in column 12 may be selectively and movablydetermined either manually or using a motorized arrangement which is perse known. X-ray film 22 is disposed beneath diffraction grid 19 in filmholder 20 through a door in the end face of the film holder.

While the illustrative embodiments provided herein refer to mammographyequipment that generates X-ray films, it will of course be understood byone familiar with radiology that digital (filmless) X-ray systems ordigitized X-ray film could be employed as well. It is sufficient forpurposes of practicing the present invention that X-ray radiationemitted from an X-ray source pass through biological tissue and form animage in a receptor, whether an X-ray film or a digital X-ray receptor.Commercially available mammography equipment that may be augmented inaccordance with the present invention includes, for example, the Contoursystem by Bennett X-Ray Technologies, Inc., Copiague, New York, theAVIVA system available from Kramex, Saddle Brook, New Jersey, and theLORAD DSM system, available from Lorad, Danbury Connecticut.

In addition to the above-described components of sonomammographyapparatus 10 that are common to previously known mammography systems,the apparatus of the present invention includes compression plate 15 andultrasonic transducer 16 movably supported on gantry 17. As shown inFIGS. 1 and 2, compression plate 15 includes gel pad 23 disposed fromthe underside of the compression plate, for example, by polyethylene bag24. Compression plate 15 may include fenestrations (not shown) forconducting biopsies of the patient's tissue. Alternatively, dependingupon the composition of the gel pad, gel pad 23 may be used withoutpolyethylene bag 24 and may include a tacky or adherent surface toassist in positioning the breast.

Gel pad 23 contacts the frontal area of the patient's breast, i.e., thenipple area, to ensure proper transmission of acoustic waves fromtransducer 16 to the distal-most portion of breast tissue 100 with aminimum of impedance mismatch. As seen in FIGS. 1 and 2, gel pad 23conforms to the distal-most portion of the breast to minimize impedancemismatch and acoustic reflectance at the gel pad/breast interface.Accordingly, gel pad may comprise an agar gelatin and water compositionor other suitable rheostaticmaterial, for example, the gelatinouselastomeric compositions described in U.S. Pat. Nos. 4,369,284,4,618,213 and 5,262,468. For sanitary purposes, gel pad 23 (andpolyethylene bag 24, if used) may be disposable, and therefore removablyattached to compression plate 15.

Referring now to FIGS. 3 and 4, another advantage of the gel pad of thepresent invention, when used in conjunction with a conventionalmammography system, is described. Referring to FIG. 3A, a portion of apreviously known X-ray mammography system compresses breast 104 betweenstandard compression plate 91 and bottom plate 92 to a uniform thickness105. As is per se known, the X-ray exposure is set to provide properexposure of thickness 105. By properly exposing the uniform thickness,however, the outer edges of breast 104, including the nipple region, aretypically overexposed, as reflected at region 106 in illustrative X-rayimage 93. To compensate for this effect, it is typical for a radiologistto take a second exposure of breast 104 at a lower X-ray dosage, thusproviding an X-ray image wherein the outer edges of the breast areproperly exposed, but the uniform thickness 105 is then underexposed.

Referring now to FIGS. 4A and 4B, an important advantage of the presentinvention is illustrated. Applicants have determined that gel pad 23 notonly provides acoustic coupling when used in a sonomammographic systemas described hereinbelow, but that gel pad 23 provides an X-rayattenuation ability as well. In FIG. 4A, the portion of the system ofFIG. 3A is shown, but also including gel pad 23 of the presentinvention. Gel pad 23 conforms or adjusts itself to those thinner partsof the breast at the outer edges where standard radiation doses causeover-exposure.

When gel pad 23 is constructed of a material having an X-ray attenuationclose to that of human tissue, the gel pad attenuates X-rays as if itwere part of the uniform thickness of breast tissue. As shown in FIG.4B, the outer edges of the breast, including the nipple region, in theresulting X-ray image 94 are no longer over-exposed. Gel pad 23 cantherefore be used to enhance conventional mammography equipment byenabling the radiologist to obtain an X-ray image having properlyexposed detail at the peripheral edges of the subject anatomy with anoverall reduction in X-ray dosage to the patient.

In addition, because gel pad 23 conforms to the shape of the patient'stissue, it distributes the force applied by compression plate 15 over alarger surface area, thus reducing the compressive stress applied to thetissue and reducing patient discomfort. Moreover, if gel pad 23 includesa slightly tacky or adherent surface, it will better grip the patient'stissue and reduce difficulties in positioning the tissue.

Referring again to FIGS. 1, 2 and 5, in the first embodiment of thepresent invention, compression plate 15 comprises a high performanceacoustically transparent ("sonolucent") and X-ray transparent("radiolucent") film which is sufficiently rigid to serve as acompression plate at a thickness of about 25 micron (1 mil). Inparticular, it is preferred that compression plate 15 have sufficientrigidity so that the local slope of the plate, under load, does notexceed one degree from the horizontal within the scan area. For furtherrigidity, compression plate 15 may include metal reinforcing bars 15along its lateral end faces.

Kapton® manufactured by E. I. Du Pont de Nemours and Company,Wilmington, Del., is a suitable material for practicing the presentinvention, as it provides both the needed sonolucent/radiolucentqualities as well as the needed rigidity to provide satisfactorily as acompression plate. In particular, a 1 mil (25 micron) thickness ofKapton®, when used as a compression plate, is expected to cause lessthan 3 dB transmission loss in acoustic energy, while providing atensile strength equivalent to that of a 2 mm thick polycarbonate plate.In addition, Kapton® is unaffected by exposure to X-ray radiation.

Other materials suitable for use in making a radiolucent and sonolucentcompression plate include Surlyn® ionomers, such as Surlyn® 8940,available from E. I. Du Pont de Nemours and Company, Wilmington,Delaware, and polymethyl pentenes, such as TPX® MX-002 and MX-004,available from Mitsui & Co., Tokyo, Japan. Plates of these materialsapproximately 6.4 mm (0.25 inch) thick are expected to be sufficientlyrigid to meet the above-defined deflection criterion if properlysupported by a stiffening frame around their periphery. In FIG. 5compression plate 15 is shown comprising a 6.4 mm (0.25 inch) thicksheet of TPX® 95 fastened to a metal frame 96. Three sides of the TPX®sheet 95 are fastened to the metal frame 96 by suitable fasteners, suchas staggered screws 97, and the fourth side is bonded into a groove 98in the frame 96. Of the two materials, the polymethyl pentenes, and TPX®in particular, are preferred due to their lower acoustic attenuation andimpedance and higher strength. A sheet made of a Surlyn® ionomer canalso be used in a similar fashion although it is softer and the acousticlosses are approximately double that of TPX®.

Referring now to FIGS. 6A and 6B, ultrasonic transducer 16 may comprisea single piston, annular or phased array imaging device of conventionaldesign. Such array devices may permit beam-focusing of ultrasonic energyto provide high resolution images of the internal structures of apatient's tissue. Ultrasound transducer 16 combines both transmit andreceive functions that are switched, respectively, between transmittingand receiving operational modes at selected times by control circuitry.

Because the internal structure and operation of ultrasonic apparatus isper se known, the specific internal configuration of that apparatusforms no part of the present invention. Transducer 16 preferablyoperates in a range of about 2 to 15 MHz. More preferably, the signalproduced by the transducer in the transmit mode is a 10 MHz burst havinga 100% bandwidth. To improve the transfer of acoustic energy, transducer16 may in addition be acoustically coupled to the upper surface ofcompression plate 15 using an appropriate coupling agent such as, forexample, glycerol, or an additional thin gel pad disposed atopcompression plate 15 (omitted for clarity from FIG. 1).

With respect to the illustrative embodiment of transducer 16 shown inFIGS. 6A and 6B, apparatus for applying a lubricating/coupling agentbetween ultrasonic transducer 16 and compression plate 15 is described.Transducer 16 is surrounded by a skirt or cover 110 that includes aspacer 11 formed along its lower edge. Spacer 111 lifts the contactsurface of transducer 16 about 0.06 mm (2.5 mils) above the surface ofcompression plate 15, and is shaped to optimize lubrication and acousticcoupling. A sponge-like material 112 dampened with a suitablelubricating/coupling fluid, for example, a water-based solution ofsurfactant and detergent, is disposed around the transducer 16 such thatthe sponge-like material 112 and the spacer 111 are in contact withcompression plate 15 at substantially the same time. Thus, as thetransducer assembly moves along the surface of compression plate 15 athin film 113 of the lubricating/coupling fluid is deposited on theplate. Cover 110 also permits the transducer assembly to be handledwithout contacting material 112.

Referring again to FIGS. 1 and 2, gantry support 18 is verticallypositioned along column 12 using a motorized or manually adjustablemechanism. Gantry support 18 includes arms 18' disposed above thelateral edges of compression plate 15. Gantry support 18 movablysupports gantry 17 for movement in distal and proximal directions "A"and "B", using a motorized track or cable arrangement 25. Gantry support18 moves gantry 17 in precise increments in the distal and proximaldirections. During X-ray exposure of the patient's tissue, gantry 17 ismoved to a distal-most position in direction "A" so that it does notinterfere with the mammogram exposure. Alternatively, gantry 17 andgantry support 18 may be hinged to swing away from the compressionplate, thus providing clear access for an X-ray exposure.

Gantry 17 (shown by dotted lines in FIG. 7) in turn comprises carriage26 that supports ultrasonic transducer 16. Gantry 17 includes its ownmotorized drive means 27 for moving carriage 26 laterally in directions"C" and "D".

Illustrative embodiments of drive means 25 and 27 are described withrespect to FIG. 7. Drive means 25 of gantry support arm 18 may comprisecables 30 that extend through arms 18' of gantry support 18. Cables 30are captured on pulleys 31 and drive wheels 32 to form upper and lowerflights 30A and 30B, respectively. Drive wheels 32 are synchronouslydriven by motor 33. Gantry 17 is fixedly connected to the upper flightsof cables 30 at points 34, so that when the upper flights of cables 30move in directions "A" and "B", gantry 17 translates in thecorresponding direction. Motor 33 is of a type that enables exactpositioning of gantry 17, for example, so that the gantry 17 can bemoved in the proximal and distal directions in precise increments, suchas 1 to 10 mm.

Still referring to FIG. 7, gantry 17 includes its own cable arrangement27 for precisely positioning carriage 26 and transducer 16. Inparticular, in the illustrative embodiment shown, cable 35 runs on drivewheel 36 and pulley 37 to form upper and lower flights 35A and 35B,respectively. Carriage 26 is fixed to lower flight 35B of cable 35 atpoint 35' so that carriage 26 moves in directions "C" and "D" inresponse to movement of lower flight 35B. Motor 38, which is supportedon gantry 17, enables precise control of carriage 26 and thus transducer16.

Alternatively, a toothed belt and gear arrangement may be substitutedfor the cables, pulleys and drive wheels of the above-describedillustrative embodiment. As further alternatives, drive means 25 and 27may employ, for example, a conventional motorized track, a threadedblock carried on a threaded drive rod controlled by an encoder andstepper motor, or any other suitable means.

It is to be understood that appropriately programmed control circuitryis provided for use with any of the foregoing drive means 25 and 27 sothat the drive means pauses at predetermined locations during transitfor a period sufficient to obtain an ultrasound image of the breasttissue at that location. In addition, gantry 17 and gentry support 18may provide release mechanisms that enable transducer 16 to be manuallypositioned by the operator.

As shown in FIG. 2, arm 18' of gantry support 18 includes slot 39,through which an extension of gantry 17 projects to engage biopsy needleguide 21. Thus, as gantry 17 moves in distal and proximal directions "A"and "B" biopsy needle guide 21 remains in alignment with ultrasonictransducer 16. Biopsy needle guide 21 includes a needle support element40 having an aperture through which a biopsy needle may be inserted toperform an ultrasound image-guided biopsy of the patient's tissue.Needle support element 40 may be positioned at any desired position bythe medical practitioner and then engaged with biopsy needle support 21for performing image-guided biopsy.

Lateral alignment of the biopsy needle in accordance with this aspect ofthe present invention provides important psychological benefits to thepatient. Since the biopsy needle is laterally inserted into thepatient's breast, rather than through the upper surface, it produces noscarring on the upper surface of the breast. Accordingly, the patientwill not be discouraged from wearing clothing (e.g., an evening gown)which exposes the upper surface of the breasts, due to concern thatunsightly scar tissue from a biopsy puncture will be visible.

Ultrasound transducer 16 generates an image corresponding to theinternal structure of the tissue located in the plane perpendicular totransducer at each of the locations where carriage 26 stops during itstransit across compression plate 15. The images or frames generated ateach of these locations is stored on a microprocessor based workstation41, such as shown in FIG. 8, for later postprocessing and manipulation.

Referring now to FIG. 8, for an embodiment of the present invention foruse with conventional mammography apparatus that generates an X-rayfilm, an X-ray film 42 is positioned on digitizing tablet 43 so thatindex marks 44 and 44' on the X-ray film coincide with positioning markson digitizing tablet 43. Digitizing tablet 43 includes pen 45 and isconnected to workstation 41 having monitor 46. Workstation 41 includessuitable software for interpreting movement of pen 45 with respect todigitizing tablet 43.

When X-ray film 42 is aligned on digitizing pad 43, pen 45 of thedigitizing tablet enables the medical practitioner to display on monitor46 the orthogonal ultrasound image corresponding to a location on X-rayfilm 42 by touching pen 45 to digitizing tablet 43. Thus, the positionof the contact of pen 45 to digitizing tablet 43 automatically brings upthe corresponding orthogonal ultrasound frame at that location,providing the medical practitioner with a holographic, i.e.,three-dimensional, view of the internal structure of the tissue.Moreover, the precise geometric registration of the ultrasound imageframes and the X-ray film provided by the present invention enables themedical practitioner to manipulate the ultrasound images, to perform,for example, digital subtraction, thereby enhancing breast lesiondetection capability.

The PowerPC® commercially available from Apple Computer, Cupertino,California, provides a suitable workstation for use as described above,while the HiSketch series of digitizing tablets, available from KyeInternational Corp., Ontario, Calif., provide suitable digitizingtablets for use in conjunction with the sonomammography apparatus of thepresent invention. Alternatively, a conventional X-ray film could bedigitized using a scanner, or a conventional video camera.

Referring now to FIGS. 9-11, an alternative embodiment of asonomammography apparatus 50 constructed in accordance with theprinciples of the present invention is described. Sonomammographyapparatus 50 includes base 51, upright vertical column 52, X-ray tube 53supported on vertical movable arm 54, compression plate 55, diffractiongrid 56, ultrasound transducer 57 and film holder 58. Components 50-54may constitute the elements of a conventional mammography system asdescribed hereinabove. X-ray sensitive film 59 is disposed in filmholder 58 beneath ultrasound transducer 57.

Sonomammography apparatus 50 differs from apparatus 10 describedhereinabove principally in that the sonolucent compression plate 15,transducer 16, gantry 17 and gantry support 18 are replaced by modifieddiffraction grid 56 and ultrasound transducer 57. Compression plate 55may be fenestrated to enable the medical practitioner to performultrasound-image guided biopsies.

Referring now to FIG. 11, diffraction grid 56 comprises an-array of anX-ray absorptive material 61, such as lead, having its interspacesfilled with a non-absorptive material 62, such as aluminum or an organicmaterial. This arrangement, which is conventional for mammographysystems, permits those X-rays which are perpendicular to the plane ofdiffraction grid 56 to pass through interspaces 62, while the array oflead lines 61 absorbs most of the diffuse radiation caused by scatteringof the X-rays as they pass through the patient's tissue 101. Diffractiongrid 56 differs from previously known devices, in that the lowersurfaces of interspaces 62 extend below the lower surfaces of lead lines61 by about 1 mm. The spaces between the extended interspaces therebycreate air pockets that serve as an acoustic absorber between ultrasonictransducer 57 and lead lines 61.

Ultrasonic transducer 57 serves the same purpose as ultrasoundtransducer 16 of the embodiment of FIGS. 1-7, namely, to alternativelysend and receive acoustic energy. Ultrasonic transducer 57 comprises atwo-dimensional array of piezoelectric linear or phased arrays 63 spacedin parallel relation. Arrays 63 may have their axes aligned orthogonallywith the lead lines of diffraction grid 56, as shown in FIG. 11, or mayhave their axes aligned with interspaces 62. Each of the arrays 63comprises a multiplicity of ultrasonic transducers elements 63' that canbe individually and sequentially activated. Spacing 64 between arrays63, which may be for example 1 cm, determines the spacing betweenadjoining frames of the ultrasound images provided by transducer 57.This resolution, as well as elevational focusing, can be improved byproviding suitable circuitry for focusing the acoustic energy emitted bymultiple ultrasonic transducer elements 63', i.e., by activatingelements in adjacent rows.

Each of ultrasonic transducer elements 63' is connected to an ultrasoundcontroller circuit, described hereinafter, by a series of connectingwires (not shown in FIG. 11). The connecting wires are routed across thetwo-dimensional array so that they coincide with the rows of X-rayabsorptive material in diffraction grid 56. By so arranging theconnecting wires to ultrasonic transducer elements 63', the connectingwires will not create images on the X-ray film during exposure of thatfilm.

Upper surfaces 65 of ultrasonic transducer elements 63' are acousticallycoupled to interspaces 62 of diffraction grid 56 using a suitablecoupling agent, for example, glycerol. Acoustic energy emitted byultrasonic transducer elements 63' is transmitted through theinterspaces of diffraction grid 56 and into tissue disposed betweenupper compression plate 55 and diffraction grid 56. A gel pad, such asthat described above with respect to the embodiment of FIGS. 1-7 may beused in conjunction with compression plate 55 and diffraction grid 56 toreduce the acoustic impedance mismatch at the interface between thediffraction grid and the distal-most portion of the patient's breasttissue 101.

Referring still to FIG. 11, arrays 63 comprise a series of layersincluding a piezoelectric material, such as copolymers of vinylidenefluoride (VDF) and trifluoroethylene (TrFE), for example, available fromToray Industries, Kamakura, Japan. Use of such materials to formultrasonic transducers is described in Ohigashi et al., "Piezoelectricand Ferroelectric Properties of P(VDF-TrFE) Copolymers And TheirApplication To Ultrasonic Transducers", page 189 et seq., in MEDICALAPPLICATIONS OF PIEZOELECTRIC POLYMERS (Galetti et al. editors), Gordonand Breach Science Publishers S. A. (1988), which is incorporated hereinby reference. The inventors have determined that a layer of gold platedcopolymer material of about 25 microns (1 mil) is practicallytransparent to X-ray (and ultrasound), the change in the received signalwhen the copolymer film is inserted between the X-ray source and thefilm being less than 1 dB.

As shown in FIG. 11, arrays 63 may form a phased array. An example of aintegrated-silicon VDF-TrFE acoustic transducer array demonstrated foruse diagnostic imaging is described in Ohigashi et al. above. Sucharrays exhibit a low degree of array element cross-coupling, may beeasily fabricated in high density, and provide excellent acousticimpedance matching to biological tissue.

Still referring to FIG. 11, ultrasonic transducer 57 comprises thinmetal backing plate 66 covered piezoelectric film 67 of a suitablematerial described hereinabove, for example, a copolymer of VDF andTrFE. Piezoelectric film 67 is in turn covered by electrode element 68,and carries on its upper surface an inactive polymer layer 69.Connecting wires (not shown) are routed to the respective electrodeelements of each of the ultrasonic transducer elements 63' so as tocoincide with the lines of X-ray absorptive material in diffraction grid56. Inactive polymer layer 69 is acoustically coupled to the lower endsof the interspace material of the diffraction grid using a suitablecoupling agent as described hereinabove.

It will be recognized by one skilled in the art of ultrasonic transducerdesign that ultrasonic transducer elements 63' of ultrasonic transducer57 can be fabricated to operate at a predetermined frequency by theselection of the thicknesses of components 66-69. Furthermore, it willbe recognized that because the acoustic signals received by the arraysduring receiver operation may include a strong reflection from the lowersurface of the X-ray absorptive grid of diffraction grid 56 (i.e., verystrong impedance mismatch), it may be necessary to filter the echosignals to eliminate this artifact. For example, echo signals obtainedusing a water path may be stored in the filtering circuitry and thensubtracted from the echoes received by the ultrasonic transducer duringactual operation.

In addition, it will be understood that by employing suitable circuitryfor controlling activation of the ultrasonic transducer elements, onlythose transducer elements corresponding to a predetermined location maybe activated. Thus, by employing a biopsy needle support, such as thatshown in FIG. 1 with an appropriate mechanism for aligning the supportwith the ultrasonic transducer elements of interest, the medicalpractitioner may perform a biopsy guided by ultrasonic images, just asfor the embodiment described in FIGS. 1-7.

Referring now to FIG. 12, another alternative embodiment of thesonomammography apparatus of the present invention is described.Sonomammography apparatus 70 includes the basic elements of amammography system as described hereinabove, including upright verticalcolumn 71, compression plate 72, diffraction grid 73, film holder 74 andX-ray sensitive film 75, and ultrasound transducer 76. In thisembodiment, compression plate 72 need not be sonolucent, sinceultrasonic transducer 76 is positioned between the compression plate andthe diffraction grid. Gel pad 77 affixed to compression plate 72 ensuresacoustic coupling of ultrasound transducer 76 to the biological tissue102.

Unlike the gantry of the embodiment of FIGS. 1-7, ultrasound transducer76 is mounted on a horse-shoe-shaped gantry 78, so that the transducerfollows a curved path as it translates along gantry 78. Ultrasoundtransducer 76 moves in small angular increments, for example, 1 to 3degrees, as it traverses the length of gantry 78.

It will be recognized by one skilled in the art of ultrasonic transducerdesign that this third arrangement provides a greater depth for theacoustic energy to penetrate in comparison to embodiments describedhereinabove. Consequently, it may be necessary to employ lower frequencytransducers for this embodiment than would be used in the previouslydescribed embodiments. For most superficial lesions, however, it isexpected that a high frequency transducer would still providesatisfactory performance.

Referring now to FIG. 13, ultrasound circuit 80 for imaging a patient'stissue is described. Circuit 80 includes ultrasonic transducer 81, motorcontroller 82, microprocessor 83 run by system software 84, receivingcircuit 85, transmit/receive switch 86, drive circuit 87, analog todigital converter 88, system storage 89 and display 90.

Transducer 81 is energized by drive circuit 87 to emit ultrasonicsignals. Once the transducer has emitted acoustic energy for a suitableperiod, the transducer is switched to receiving mode. As transducer 81responds to the echoes of the emitted signals, it generates electricalsignals in receiving circuit 85.

Receiving circuit 85 preferably has a wide dynamic range, for example,100 dB, to enable high contrast resolution. Since the receiving circuitrecords the transmitted pulse as well as the returning echoes, the firstT₀ microseconds corresponding to the time-off-light from the transducersurface to the tissue is ignored. Receiving circuit 85 also includes anautomatic gain amplifier that can be adjusted to compensate for theattenuation of the returning signal. The received signal is thereforeamplified and processed by receiving circuit 85 before being fed toanalog-to-digital converter circuit 88. Analog-to-digital convertertranslates the analog electrical echo signals into digital signals.These digitally encoded ultrasound images are in turn stored in systemstorage device 89.

Microprocessor 83 monitors motor controller 82, which in turn controlsthe movement of the ultrasonic transducer (for example, movement ofgantry 17 and gantry support 18 in the embodiment of FIGS. 1-7) andcontinuously computes the position of transducer 81. The digitized datacorresponding to the gantry location at each ultrasound image locationis stored in system storage 89 together with the ultrasound image atthat location.

Alternatively, because the digitized data collected after each pulse isstored in system storage device 89 in a consecutive manner, and thepropagation path for either electronic or mechanical steering can bepredetermined, the orientation and position of transducer 81 may bedirectly correlated with the location of the digital data stored insystem storage 89.

It is known to use ultrasonic signals for the assessment of tissuevasculature by estimating the frequency or temporal shift due to bloodflow through the imaged tissue. Such systems, which are based on theDoppler principle, are described in Baker, "Pulse Ultrasound DopplerBlood Flow Sensing", IEEE Transactions on Sonics and Ultrasonics, Vol.SU-17, No. 3 (1970). Data related to blood flow may also therefore beacquired using ultrasound transducer 81, which data may be processed andstored in system storage 89 together with the echo data.

In addition, because blood flow creates a speckle effect in theultrasound image, it may be desirable to transmit several pulses at eachimaging location and then use standard noise reduction techniques toaverage out the speckle effect caused by blood flow. Also, the variationin speckle due to the motion of the transducer enables severalconsecutive acquisitions of the return echo to be averaged to reduce thespeckle. Digital subtraction of the data received from a water path andmost probably due to reverberations could also be subtracted from thedigitized data to improve the ultrasound image.

For an embodiment of the present invention such as that shown in FIGS.9-11, microprocessor 83 may control the sequential operation of theindividual ultrasonic transducer elements 63' of the two-dimensionalultrasonic transducer 57. The location of the ultrasound images instorage system 89 may be used to correlate those images with specificlocations in the phased array, as described above.

System software 84, which may reside in a conventional microprocessorbased workstation, enables data stored in storage device 89 to bemanipulated so that holographic views may be generated and viewed fromdifferent angles. In addition, the software may enable viewing of aparticular region of interest determined relative to the radio-opaquelines (not shown in FIGS. 1 or 9) provided on the compression plate orin accordance with the position of the pen of the digitizing tablet, asdescribed above with respect to FIG. 8. Images are displayed on displaydevice 90.

Set-up and operation of the sonomammography apparatus of the presentinvention is straight forward, and can be accomplished by a singleoperator. The medical practitioner or operator positions the breast formammographic studies in conventional fashion. Following (or before) theX-ray exposure, the ultrasound transducer is activated to image thebreast tissue at discrete locations, with the ultrasound images beingstored for review on the workstation.

Since cross-sectional views of the entire breast are stored, theresulting data may be manipulated, either individually or in conjunctionwith a digitized X-ray image, in accordance with the methods describedhereinbelow.

The present invention further includes methods of obtaining and viewingultrasound images and an X-ray image/geometrically registered ultrasoundimage of biological tissue. A first method of obtaining a ultrasoundimage and geometrically registered X-ray image comprises the steps of:

(a) immobilizing the biological tissue with respect to a referencepoint;

(b) exposing the biological tissue to X-rays to generate an X-ray filmof the internal structure of the biological tissue;

(c) without any intervening movement of the biological tissue withrespect to the reference point, coupling an ultrasonic transducer to thebiological tissue to generate a plurality of the ultrasound images ofthe biological tissue; and

(d) correlating the plurality of ultrasound images with predeterminedlocations on the X-ray film.

It will of course be understood that steps (b) and (c) of exposing thetissue to X-ray radiation and conducting the ultrasound scanning may bereadily interchanged as needed in a particular application.

The methods of the present invention also include the steps ofprocessing, storing and manipulating ultrasound images to enhance thediagnostic capabilities of the stored images, using, for example, noisefiltering or digital subtraction techniques.

Referring to FIGS. 1 and 14, a first method of viewing the storedultrasound image data acquired with the apparatus of the presentinvention is described. As shown in the uppermost portion of FIG. 14, animaginary three dimensional coordinate system 120, consisting of X, Y, Zdirections, can be imposed on the apparatus of FIG. 1 so that the X-Yplane coincides with the surface of lower compression plate 19, and theZ axis corresponds to elevation. In coordinate system 120, ultrasonictransducer 16 provides image "a" of the breast interior in the X-Z planeas it scans along upper compression plate 15 in directions C-D. Asultrasonic transducer 16 is moved in directions A-B, it generatesadditional frames in the X-Z plane, indicated in FIG. 14 as "b" and "c".

Since cross-sectional views in the X-Z plane are stored for the entirebreast, it is possible to sum each propagation line and obtain atwo-dimensional projection map of the breast attenuation for use inbreast cancer screening. In particular, in accordance with a firstmethod of the present invention, the data stored in each frame "a"through "c" shown in FIG. 14 may be summed in the Z direction to providea single line in the X-Y plane, thus generating a two dimensionalultrasound image 121. By projecting the ultrasound data in the X-Z planeinto a single line in the X-Y plane to create image 121, tissueabnormalities (indicated by x's in FIG. 14) can be displayed in the sameformat as digitized conventional mammogram 122 obtained with the X-rayportion of apparatus 10 as described hereinabove.

When an ultrasound image 121 as obtained above is then overlaid on adigitized X-ray image 122, applicants have observed tissue abnormalitiescan be readily isolated and identified. In addition, applicants haveobserved that color coding ultrasound image 121 and X-ray image 122speeds this identification process.

In another method in accordance with the present invention, thecross-sectional views "a"-"c" may also be displayed as a threedimensional representation of a region of interest, for example, for usein analyzing Doppler or vasculature data. An alternative presentation ofthe data might consist of a loop of consecutive frames.

Yet another method of viewing and analyzing data acquired with theapparatus of the present invention employs the principle that theacoustic backscattering of tissue is a function of density andcompressibility. Applicants have determined that a non-linearrelationship with respect to compression exists for malignant tissue. Inparticular, applicants have discovered that tissue abnormalities tend tobe stiffer and less compressible than healthy tissue. These resultssuggest that tumor detection may be enhanced by compression of thebreast tissue and the use of digital subtraction techniques to isolatesuspicious lesions.

In accordance with this method of the present invention, the patient'stissue is first compressed using compression plate 15 and gel pad 23with a first force F₁. Ultrasonic transducer 16 is then activated togenerate a first scan of the tissue and the data is stored as describedhereinabove. The force applied to the patient's tissue is then changedto a new level F₂, which may be greater or less than F₁, and a secondultrasound scan of the tissue is performed and stored. The resultingdata for the two compression levels is digitally subtracted, and theresults displayed in a three-dimensional or two-dimensional format asdescribed above. Applicants have observed that, due to the lowercompressibility of lesions relative to healthy tissue, the lesions arewell-defined in the composite image.

In accordance with yet another method of the present invention, theknowledge of the relative position of a tissue segment in both breastsallows the use of digital subtraction techniques using the digitizedultrasound images to isolate suspicious lesions. For example, theultrasound image frames from similar planes in both breasts may bedigitally subtracted and the difference in intensities summed. Based ona predetermined threshold, only images that are deemed to substantiallydifferent, using that test, are presented for review by the medicalpractitioner.

With respect to gel pad 23 of the first embodiment describedhereinabove, the present invention also encompasses a method ofenhancing X-ray images obtained by previously known X-ray equipment,comprising the steps of:

(a) immobilizing the biological tissue with respect to a referencepoint;

(b) providing a gel pad that conforms to the breast under study, the gelpad having an X-ray attenuation capability similar to that of humantissue; and

(c) exposing the biological tissue to a single X-ray dosage to generatean X-ray film of the internal structure of the biological tissue that issubstantially entirely properly exposed, even near the outer edges ofthe breast.

It will be understood that the foregoing is merely illustrative of theapparatus and methods of the present invention, and that variousmodifications can be made by those skilled in the art without departingfrom the scope and spirit of the invention.

What is claimed is:
 1. In apparatus for obtaining images of biologicaltissue by passing X-ray radiation through a biological tissue to form animage in a receptor, the apparatus comprising an X-ray source foremitting X-ray radiation, first and second compression surfaces adaptedfor immobilizing the biological tissue therebetween, and a receptordisposed adjacent the second compression surface, the X-ray sourcedisposed adjacent the first compression surface so that X-ray radiationemitted from the source passes through the biological tissue and isreceived by the receptor, the improvement comprising:a compression platethat is radiolucent and sonolucent, the compression plate having firstand second surfaces, the second surface forming the first compressionsurface; an ultrasonic transducer disposed adjacent to the first surfaceof the compression plate; and drive means for moving the ultrasonictransducer across the first surface of the compression plate while thebiological tissue remains immobilized between the first and secondcompression surfaces.
 2. The apparatus as defined in claim 1 wherein thecompression plate comprises a material selected from the groupconsisting of Kapton®, a Surlyn® ionomer, and a polymethyl pentene. 3.The apparatus as defined in claim 2 wherein the polymethyl pentene isTPX®.
 4. The apparatus as defined in claim 2 wherein the material has aperiphery and the material is coupled around the periphery to a rigidframe.
 5. The apparatus as defined in claim 1 further comprising a gelpad for acoustically coupling a portion of the biological tissue to theultrasonic transducer.
 6. The apparatus as defined in claim 5 whereinthe biological tissue has an X-ray attenuation characteristic, the gelpad is disposed between the first and second compression surfaces and incontact with the biological tissue, and the gel pad comprises a materialthat conforms to the shape of the biological tissue and has an X-rayattenuation characteristic near the X-ray attenuation characteristic ofthe biological tissue.
 7. The apparatus as defined in claim 6 whereinthe gel pad reduces scattering of the X-ray radiation relative toscattering of the X-ray radiation in air.
 8. The apparatus as defined inclaim 5 wherein the biological tissue comprises a portion of a patientand has a non-uniform shape and a surface area, the compression plateimposes a force on the biological tissue to compress the biologicaltissue to a uniform thickness, and the gel pad conforms to thenon-uniform shape to distribute the force over the surface area andreduce discomfort in the patient.
 9. The apparatus as defined in claim 8wherein the gel pad comprises an adherent surface that assists inpositioning the biological tissue between the upper compression surfaceand the lower compression surface.
 10. The apparatus as defined in claim1 further comprising lubricating means for providing a film of fluidbetween the ultrasonic transducer and the compression plate to lubricateand acoustically couple the ultrasonic transducer to the compressionplate.
 11. The apparatus as defined in claim 1 wherein the drive meansfurther comprises:a gantry support; a gantry movably engaged with thegantry support for movement in the distal and proximal directions; acarriage movably engaged with the gantry for lateral movement.
 12. Theapparatus as defined in claim 11 wherein the drive means furthercomprises:a first motorized cable arrangement for driving the gantryalong the gantry support; a second motorized cable arrangement fordriving the carriage along the gantry; and circuitry for controllingoperation of the first and second motorized cable arrangements.
 13. Theapparatus as defined in claim 1 further comprising:a biopsy instrumentsupport; means for aligning the biopsy instrument support with theultrasonic transducer so that a medical practitioner may perform abiopsy guided by the plurality of ultrasonic images.
 14. In apparatusfor obtaining images of biological tissue by passing X-ray radiationthrough a biological tissue to form an image in a receptor, theapparatus comprising an X-ray source for emitting X-ray radiation, firstand second compression surfaces adapted for immobilizing the biologicaltissue therebetween, and a receptor disposed adjacent the secondcompression surface, the X-ray source disposed adjacent the firstcompression surface so that X-ray radiation emitted from the sourcepasses through the biological tissue and is received by the receptor,the improvement comprising:an ultrasonic transducer disposed adjacentthe second compression surface, the ultrasonic transducer beingradiolucent; a coupling medium that acoustically couples the ultrasonictransducer to the second compression surface; and control circuitry foractivating the ultrasonic transducer to generate a plurality ofultrasound images of the biological tissue while the biological tissueremains immobilized between the upper and lower compression surfaces.15. The apparatus as defined in claim 14 further comprising a gel padfor acoustically coupling a portion of the biological tissue to theultrasonic transducer.
 16. The apparatus as defined in claim 14 whereinthe biological tissue has an X-ray attenuation characteristic, the gelpad is disposed between the first and second compression surfaces and incontact with the biological tissue, and the gel pad comprises a materialthat conforms to the shape of the biological tissue and has an X-rayattenuation characteristic near the X-ray attenuation characteristic ofthe biological tissue.
 17. The apparatus as defined in claim 16 whereinthe gel pad reduces scattering of the X-ray radiation relative toscattering of the X-ray radiation in air
 18. The apparatus as defined inclaim 14 wherein the biological tissue comprises a portion of a patientand has a non-uniform shape and a surface area, the first and secondcompression surfaces impose a force on the biological tissue to compressthe biological tissue to a uniform thickness, and the gel pad conformsto the non-uniform shape to distribute the force over the surface areaand reduce discomfort in the patient.
 19. The apparatus as defined inclaim 18 wherein the gel pad comprises an adherent surface that assistsin positioning the biological tissue between the first and secondcompression surfaces.
 20. The apparatus as defined in claim 14 whereinthe ultrasonic transducer comprises a multiplicity of piezoelectrictransducer elements.
 21. The apparatus as defined in claim 20 whereinthe control circuitry further comprises circuitry for activatingpredetermined ones of the multiplicity of piezoelectric transducerelements to provide beam forming and elevational focussing of theacoustic energy.
 22. The apparatus as defined in claim 20 wherein thecontrol circuitry comprises circuitry for activating a predeterminedplurality of the multiplicity of piezoelectric elements to generate anultrasonic image at a predetermined location, the apparatus furthercomprising:a biopsy instrument support; means for aligning the biopsyinstrument support with the predetermined plurality of piezoelectricelements so that a medical practitioner may perform a biopsy guided bythe ultrasonic image at the predetermined location.
 23. In apparatus forobtaining images of a biological tissue having a shape by passing X-rayradiation through a biological tissue to form an image in a receptor,the biological tissue having an X-ray attenuation characteristic, theapparatus comprising an X-ray source for emitting X-ray radiation, firstand second compression surfaces adapted for immobilizing the biologicaltissue therebetween so that the biological tissue has a portion ofuniform thickness and a peripheral portion of non-uniform thickness, anda receptor disposed adjacent the second compression surface, the X-raysource disposed adjacent the first compression surface so that X-rayradiation emitted from the source passes through the biological tissueand is received by the receptor, the improvement comprising:anultrasonic transducer disposed between the first and second compressionsurfaces; a gel pad disposed between the first and second compressionsurfaces and in contact with the biological tissue, the gel padacoustically coupling a portion of the biological tissue to theultrasonic transducer, comprising a material that conforms to the shapeof the biological tissue and having an X-ray attenuation characteristicnear the X-ray attenuation characteristic of the biological tissue, thegel pad enhancing the image of the peripheral portion formed in thereceptor; and gantry means for moving the ultrasonic transducer along acurved path between the first and second compression surfaces while thebiological tissue remains immobilized therebetween.
 24. The apparatus asdefined in claim 23 wherein the gel pad reduces scattering of the X-rayradiation relative to scattering of the X-ray radiation in air.
 25. Theapparatus as defined in claim 23 wherein the biological tissue comprisesa portion of a patient and has a non-uniform shape and a surface area,the first and second compression surfaces impose a force on thebiological tissue, and the gel pad conforms to the non-uniform shape todistribute the force over the surface area and reduce discomfort in thepatient.
 26. The apparatus as defined in claim 25 wherein the gel padcomprises an adherent surface that assists in positioning the biologicaltissue between the first and second compression surfaces.
 27. Apparatusfor generating a plurality of ultrasound images of a biological tissue,the apparatus for use with an X-ray system that forms an image of thebiological tissue in a receptor, the apparatus comprising:a compressionplate that is radiolucent and sonolucent, the compression plate havingfirst and second surfaces, the first surface forming a compressionsurface against which the biological tissue is immobilized; anultrasonic transducer disposed adjacent the second surface; and drivemeans for moving the ultrasonic transducer across the second surfacewhile the biological tissue remains immobilized against the compressionsurface.
 28. The apparatus as defined in claim 27 wherein thecompression plate comprises a material selected for a group consistingof Kapton®, a Surlyn® ionomer, and a polymethyl pentene.
 29. Theapparatus as defined in claim 28 wherein the polymethyl pentene is TPX®.30. The apparatus as defined in claim 28 wherein the material has aperiphery and the material is coupled around the periphery to a rigidframe.
 31. The apparatus as defined in claim 27 further comprising a gelpad for acoustically coupling a portion of the biological tissue to theultrasonic transducer.
 32. The apparatus as defined in claim 31 whereinthe biological tissue has an X-ray attenuation characteristic, the gelpad is disposed against the compression surface and in contact with thebiological tissue, and comprises a material that conforms to the shapeof the biological tissue and has an X-ray attenuation characteristicnear the X-ray attenuation characteristic of the biological tissue. 33.The apparatus as defined in claim 32 wherein the gel pad reducesscattering of the X-ray radiation relative to scattering of the X-rayradiation in air.
 34. The apparatus as defined in claim 31 wherein thebiological tissue comprises a portion of a patient and has a non-uniformshape and a surface area, the compression plate imposes a force on thebiological tissue to compress the biological tissue to a uniformthickness, and the gel pad conforms to the non-uniform shape todistribute the force over the surface area and reduce discomfort in thepatient.
 35. The apparatus as defined in claim 34 wherein the gel padcomprises an adherent surface that assists in positioning the biologicaltissue relative to the compression surface.
 36. The apparatus as definedin claim 27 further comprising lubricating means for providing a film offluid between the ultrasonic transducer and the compression plate tolubricate and acoustically couple the ultrasonic transducer and thecompression plate.
 37. The apparatus as defined in claim 27 wherein thedrive means further comprises:a gantry support; a gantry movably engagedwith the gantry support for movement in the distal and proximaldirections; a carriage movably engaged with the gantry for lateralmovement.
 38. The apparatus as defined in claim 37 wherein the drivemeans further comprises:a first motorized cable arrangement for drivingthe gantry along the gantry support; a second motorized cablearrangement for driving the carriage along the gantry; and circuitry forcontrolling operation of the first and second motorized cablearrangements.
 39. The apparatus as defined in claim 37 furthercomprising:a biopsy instrument support; means for aligning the biopsyinstrument support with the ultrasonic transducer so that a medicalpractitioner may perform a biopsy guided by the plurality of ultrasonicimages.
 40. Apparatus for generating a plurality of ultrasound images ofa biological tissue, the apparatus for use with an X-ray system thatforms an image of the biological tissue in a receptor, the receptor, theapparatus comprising:a compression surface against which the biologicaltissue is immobilized, the compression surface being radiolucent; anultrasonic transducer disposed adjacent the compression surface; acoupling medium interposed between the ultrasonic transducer and thecompression surface; and control circuitry for activating the ultrasonictransducer to generate a plurality of ultrasound images of thebiological tissue while the biological tissue remains immobilizedagainst the compression surface.
 41. The apparatus as defined in claim40 further comprising a gel pad for acoustically coupling a portion ofthe biological tissue to the ultrasonic transducer.
 42. The apparatus asdefined in claim 41 wherein the biological tissue has an X-rayattenuation characteristic, the gel pad is disposed against thecompression surface and in contact with the biological tissue, and thegel pad comprises a material that conforms to the shape of thebiological tissue and has an X-ray attenuation characteristic near theX-ray attenuation characteristic of the biological tissue.
 43. Theapparatus as defined in claim 42 wherein the gel pad reduces scatteringof the X-ray radiation relative to scattering of the X-ray radiation inair.
 44. The apparatus as defined in claim 41 wherein the biologicaltissue comprises a portion of a patient and has a non-uniform shape anda surface area, the compression surface imposes a force on thebiological tissue to compress the biological tissue to a uniformthickness, and the gel pad conforms to the non-uniform shape todistribute the force over the surface area and reduce discomfort in thepatient.
 45. The apparatus as defined in claim 44 wherein the gel padcomprises an adherent surface that assists in positioning the biologicaltissue relative to the compression surface.
 46. The apparatus as definedin claim 41 wherein the ultrasonic transducer comprises a multiplicityof piezoelectric transducer elements.
 47. The apparatus as defined inclaim 48 wherein the control circuitry further comprises circuitry foractivating predetermined ones of the multiplicity of piezoelectrictransducer elements to provide beam forming and elevational focussing ofthe acoustic energy.
 48. The apparatus as defined in claim 46 whereinthe control circuitry comprises circuitry for activating a predeterminedplurality of the multiplicity of piezoelectric elements to generate anultrasonic image at a predetermined location, the apparatus furthercomprising:a biopsy instrument support; means for aligning the biopsyinstrument support with the predetermined plurality of piezoelectricelements so that a medical practitioner may perform a biopsy guided bythe ultrasonic image at the predetermined location.
 49. Apparatus forgenerating a plurality of ultrasound images of a biological tissue,including peripheral portions, the apparatus for use with an X-raysystem that forms an image of the biological tissue in a receptor, sothat when the apparatus is used with the X-ray system, the plurality ofultrasound images of the biological tissue may be correlated to theimage formed in the receptor, the X-ray system including first andsecond compression surfaces adapted for immobilizing the biologicaltissue therebetween so that the biological tissue has a portion ofuniform thickness and a peripheral portion of non-uniform thickness, thebiological tissue having an X-ray attenuation characteristic theapparatus comprising:an ultrasonic transducer disposed between the firstand second compression surfaces; a gel pad disposed between the firstand second compression surfaces and in contact with the biologicaltissue, the gel pad acoustically coupling a portion of the biologicaltissue to the ultrasonic transducer, comprising a material that conformsto the shape of the biological tissue and having an X-ray attenuationcharacteristic near the X-ray attenuation characteristic of thebiological tissue, the gel pad enhancing the image of the peripheralportion formed in the receptor; and gantry means for moving theultrasonic transducer along a curved path between the first and secondcompression surfaces while the biological tissue remains immobilizedtherebetween, so that the ultrasonic transducer generates a plurality ofultrasound images of the biological tissue that may be correlated to theimage formed in the receptor.
 50. The apparatus as defined in claim 49wherein the gel pad reduces scattering of the X-ray radiation relativeto scattering of the X-ray radiation in air.
 51. The apparatus asdefined in claim 49 wherein the biological tissue comprises a portion ofa patient and has a non-uniform shape and a surface area, the first andsecond compression surfaces impose a force on the biological tissue, andthe gel pad conforms to the non-uniform shape to distribute the forceover the surface area and reduce discomfort in the patient.
 52. Theapparatus as defined in claim 51 wherein the gel pad comprises anadherent surface that assists in positioning the biological tissuebetween the first and second compression surfaces.
 53. A method ofobtaining an X-ray image of a biological tissue and an ultrasound imageof the biological tissue that may be correlated to the X-ray image,comprising a series of steps of:(a) immobilizing the biological tissuewith respect to a reference point using a compression plate that isradiolucent and sonolucent; (b) exposing the biological tissue to X-rayradiation to generate an X-ray image of the biological tissue in areceptor; (c) acoustically coupling an ultrasound transducer to thecompression plate to generate a plurality of ultrasound images of thebiological tissue without intervening movement of the biological tissuewith respect to the reference point; and (d) displaying any one of theplurality of ultrasound images corresponding to a predetermined locationon the X-ray image.
 54. The method as defined in claim 53 wherein step(b) is performed after step (c).
 55. The method as defined in claim 53further comprising steps of:(e) repeatedly generating and displaying aplurality of ultrasound images on a timewise basis; (f) inserting abiopsy instrument into the biological tissue so that a portion of thebiopsy instrument is visible in the plurality of ultrasound images; and(g) maneuvering the biopsy instrument to a desired location within thebiological tissue based on the X-ray image and the plurality ofultrasound images.
 56. The method as defined in claim 53 furthercomprising steps of:(e) storing the plurality of ultrasound images in astorage medium; and (f) retrieving any one of the plurality ofultrasound images from the storage medium corresponding to apredetermined location on the X-ray image.
 57. The method as defined inclaim 53 further comprising a step of processing the plurality ofultrasound images to enhance the diagnostic capabilities of thoseimages.
 58. The method as defined in claim 53 further comprising stepsof:(e) repeatedly generating and displaying a plurality of ultrasoundimages on a timewise basis at a location within the biological tissue;(f) processing the plurality of ultrasound images at the location toprovide an indicator corresponding to blood flow at that location. 59.The method as defined in claim 53 further comprising steps of:(e)generating a plurality of Doppler signals for the biological tissue byacoustically coupling an ultrasonic transducer to the biological tissue,without intervening movement of the biological tissue with respect tothe reference point; and (f) displaying an indicator corresponding tothe plurality of Doppler signals for a predetermined location on theX-ray image.
 60. The method as defined in claim 53 further comprising aseries of steps of:(e) storing the plurality of ultrasound images in astorage medium; and (f) displaying selected ones of the plurality ofultrasound images to provide a holographic view of the interior featuresof the biological tissue.
 61. The method as defined in claim 53 whereinthe compression plate lies in an X-Y plane and each one of theultrasound images comprises a multiplicity of digitally encoded datavalues obtained at a multiplicity of planes along a Z axis orthogonal tothe X-Y plane, the method further comprising the steps of:(e) summingthe multiplicity of digitally encoded data values along the Z axis togenerate a projection of the plurality of ultrasound images into the X-Yplane; and (f) displaying the projection.
 62. The method as defined inclaim 61 further comprising a step of comparing the X-ray image to theprojection to isolate selected ones of the interior features of thebiological tissue.
 63. The method as defined in claim 62 wherein theX-ray image and the projection are color coded.
 64. The method asdefined in claim 62 wherein step (a) is performed after step (c).
 65. Amethod of screening a biological tissue for abnormalities, comprising aseries of steps of:(a) immobilizing the biological tissue with respectto a reference point using a compression plate that is radiolucent andsonolucent; (b) exposing the biological tissue to X-ray radiation togenerate an X-ray image of the biological tissue in a receptor; (c)applying a first compressive load to the biological tissue; (d)acoustically coupling an ultrasound transducer to the compression plateto generate a first plurality of digitally encoded ultrasound images ofthe biological tissue with respect to the reference point; (e) storingthe first plurality of digitally encoded ultrasound images; (f) applyinga second compressive load to the biological tissue, the secondcompressive load different than the first compressive load; (g)operating the ultrasound transducer to generate a second plurality ofdigitally encoded ultrasound images of the biological tissue withrespect to the reference point; (h) digitally subtracting each one ofthe second plurality of digitally encoded ultrasound images from acorresponding one of the first plurality of digitally encoded ultrasoundimages with respect to the reference point; (i) displaying thedifference of first and second digitally encoded ultrasound images; and(j) comparing the difference of the first and second digitally encodedultrasound images at a predetermined location to a correspondinglocation on the X-ray image.
 66. The method as defined in claim 65wherein the first and second pluralities of digitally encoded ultrasoundimage are color coded.